In a typical automobile air conditioning system, a refrigerant is compressed by a compressor unit driven by the automobile engine. The compressed refrigerant, at high temperature and pressure, enters a condenser where heat is removed from the compressed refrigerant. The refrigerant then travels through a receiver dryer to an expansion valve. The expansion valve causes the pressure of the refrigerant to drop as it flows through the valve, which causes the refrigerant to change phase from liquid to gas form as it enters the evaporator. In the evaporator, heat is drawn from the environment to replace the latent heat of vaporization, thus cooling the environmental air. The low pressure refrigerant flow from the evaporator returns to the suction side of the compressor to begin the cycle anew.
The high pressure refrigerant flow through the expansion valve must be regulated in response to the degree of superheat in the low pressure refrigerant flow between the evaporator and suction side of the compressor to maximize the air conditioning performance. The superheat is defined as the temperature difference between the actual temperature of the low pressure refrigerant flow and the temperature of evaporation of the flow. Traditionally, the low pressure flow superheat has been sensed remotely by use of a feeler bulb. The feeler bulb is positioned in contact with the pipe carrying the low pressure refrigerant. A pressure carrier extends from the feeler bulb to a power element in the expansion valve which modulates the valve. Many of these components are affected by external thermal disturbances, reducing the accuracy of the modulation. Also, the complexity results in a long "dead time" between a superheat transient in the low pressure refrigerant flow, and a compensating regulation in the expansion valve.
Attempts have been made to improve expansion valve performance. For example, U.S. Pat. No. 3,450,345 discloses a bulbless thermostatic expansion valve. U.S. Pat. No. 3,537,645 also discloses a bulbless expansion valve. Japanese laid-open patent application No. 144,875/1984, and Japanese laid-open utility model application No. 130,378/1985, also illustrate expansion valve designs. However, the need remains for an expansion valve which maximizes performance of the system, while minimizing cost and maximizing reliability.